1. Field of the Invention
This invention relates generally to an interference compensating circuit and more particularly to a circuit for the compensation of current interference signals in another circuit to which a voltage containing a disturbing component is applied. Thus the present invention concerns the compensation of interference signals inherent in a current-dependent signal where these signals are caused, not by variations of a load impedance of the circuit, but by variations or fluctuations of the source voltage feeding the circuit or of the internal resistance of the source.
2. Description of the Prior Art
In practical applications, the point of demarcation between the external impedance of a circuit and the internal resistance or impedance of the source is in fact dependent on the measuring or monitoring task in question. In the case of monitoring a certain section of a network or line for overcurrents due to short circuits, for example, the impedance -- or if appropriate, a corresponding equivalent impedance -- of the section concerned is to be viewed as the external impedance. Fluctuations (or collapse in the event of a short circuit) of this external impedance govern the current signals to be detected or selected, whereas the equivalent impedance of the remainder of the network, insofar as it influences the current in the monitored section, is to be viewed as the internal impedance of the source feeding the section. Variations of this impedance, but more importantly of the source voltage -- in particular periodic voltage variations of a disturbing nature, such as harmonics from frequency changes, inverters or rectifiers -- give rise to current interference signals in the monitored section of network or line which require compensation.
In practice, such interference signals seriously impair the monitoring of overcurrents and short circuits. This is particularly so in the case of railway contact wires with high maximum operating currents which in themselves are still acceptable, but which under certain circumstances can be of the same order to magnitude as short-circuit currents. In such cases, also, the current fluctuations attributable to the source voltage can be of the same order as the short-circuit currents. In such cases, also, the current fluctuations attributable to the source voltage can be of the same order as the short-circuit currents, so that detection of short circuits without compensation of current variations due to the source voltage is not possible. However, the detection of short circuits must be independent of whether maximum permissible currents are exceeded, owing to the risk of damage, and especially fire, to the installations concerned.